There are two different URLs for workspaces: the private URL (the one that is currently working for you) and the public one (the IP address from the Share panel, which corresponds directly to your machine). You can use the public IP address as your Preview URL, but you will need to open up access to your host. You can find some documentation on how to do that here: https://docs.aws.amazon.com/cloud9/latest/user-guide/app-preview.html#app-preview-share